In order to save power consumption in electronic devices, highly efficient DC-DC converters are used in power supply circuits. However, a DC-DC converter generates a large switching noise when turning on or off a switching element. This noise is produced not only at the switching frequency but also at its harmonics; i.e., frequencies that are integer-multiples of the switching frequency. As a result, when a DC-DC converter and a radio transmission/reception circuit are integrated on the same semi-conductor device, for example, the switching noise of the DC-DC converter adversely affects frequencies used by the radio transmission/reception circuit.
FIG. 1 shows a conventional oscillating circuit known from Japanese Laid-Open Patent Application No. 9-266425 (“Patent Document 1”) or No. 9-266426 (“Patent Document 2”). As shown in FIG. 1, an oscillating element 56 is connected to a reference oscillating circuit 55. The reference oscillating circuit 55 generates an oscillating signal FT having a certain frequency determined by the oscillating element 56. The oscillating signal FT is frequency-divided by a frequency divider 57 to generate a signal CC that is output to a phase comparator 58. The phase comparator 58 compares the phase of the signal CC with the phase of a frequency-divided signal DC output from a frequency divider 64, generating a frequency error signal EPC, which is supplied via a low-pass filter 59 to the base of an NPN transistor 60.
The oscillator 61 includes a CR oscillating circuit and outputs an oscillating signal Fs whose frequency is set by a resistor 62 and a capacitor 63. The transistor 60 is connected in parallel to the resistor 62 so that the resistance value across the resistor 62 can be varied by the transistor 60, thus varying the frequency of the oscillating signal Fs. The oscillating signal Fs is supplied to the frequency divider 64 and a DC-DC control circuit (not shown). The frequency divider 64 divides the oscillating signal Fs at a predetermined dividing ratio that is set by a frequency-division control signal BC output from a station-selecting microprocessor in a tuner unit (not shown) for receiving a radio broadcast.
For example, suppose that the frequency divider 57 generates the signal CC of 5 kHz by dividing the oscillating signal FT while the frequency divider 64 outputs the frequency-divided signal DC by dividing the oscillating signal Fs by 20. The phase comparator 58 compares the signal CC with the frequency-divided signal DC and supplies the frequency error signal EPC to the transistor 60 so that both signals can have the same frequency. Thus, the frequency of the oscillating signal Fs generated by the oscillator 61 is 100 kHz. Because the DC-DC control circuit switches the switching transistor based on this oscillating signal Fs, a noise component is generated at 100 kHz and at its integer-multiple harmonics.
When the tuner unit (not shown) is operated to receive a broadcast radio wave of 999 kHz, for example, the dividing ratio of the frequency divider 64 is set to 21 by the frequency-division control signal BC. Then, the oscillating signal Fs is divided by 21, so that the frequency of the frequency-divided signal DC becomes about 4.76 kHz. The phase comparator 58 outputs the frequency error signal EPC in order to increase the frequency of the oscillating signal Fs so that the frequency of the frequency-divided signal DC becomes 5 kHz. Specifically, the frequency of the oscillating signal Fs is increased to 105 kHz so that the frequency-divided signal DC of 5 kHz can be obtained with the dividing ratio of 21. Because the switching transistor is driven on the basis of the 105 kHz oscillating signal Fs, the switching noise has frequencies different from the frequencies in the reception band of the broadcast radio wave or the frequency of an intermediate frequency signal, thus preventing reception difficulties.
FIG. 2 shows another conventional oscillating circuit known from Patent Document 1 or 2. As shown, a phase comparator 65 is supplied with a reference signal CB of a predetermined frequency generated by a tuner unit (not shown), and with a frequency-divided signal DC from a frequency divider 64. The phase comparator 65 compares the reference signal CB with the frequency-divided signal DC to generate a frequency error signal EFE, which is supplied via a low-pass filter 59 to a transistor 60.
When the dividing ratio of the frequency divider 64 is set at 12, and the inter-station frequency of broadcast radio waves received by the tuner unit is 9 kHz, the reference signal CB of 9 kHz is input to the phase comparator 65. Because the phase comparator 65 generates the frequency error signal EFE so that the reference signal CB and the frequency-divided signal DC can have the same frequency, the frequency of the oscillating signal Fs generated by the oscillator 61 becomes 108 kHz. In this case, when a broadcast radio wave of 1080 kHz is received, the radio wave is affected by the switching noise because the frequency of the received broadcast radio wave is equal to a harmonic component of the switching noise. Thus, the dividing ratio of the frequency divider 64 is changed to 13 by the frequency-division control signal BC. As a result, the frequency of the oscillating signal Fs is changed to 117 kHz, so that the harmonic components of the switching noise stay out of the reception band of the received broadcast radio wave, thus preventing reception difficulties.
FIG. 3 shows another conventional oscillating circuit known from Patent Document 1 or 2. As shown in FIG. 3, an oscillating element 67 is connected to an oscillating circuit 66. The oscillating circuit 66 generates an oscillating signal Fu of a certain frequency determined by the oscillating element 67. The oscillating signal Fu is input to a frequency divider 68.
The frequency divider 68 divides the oscillating signal Fu and outputs an oscillating signal Fs that is supplied to a DC-DC control circuit (not shown) for driving a switching transistor. To the frequency divider 68, there is connected a frequency-division control unit 69 which generates a frequency-division control signal BD for changing the dividing ratio of the frequency divider 68 at certain time intervals con-tinuously or discontinuously.
Because the dividing ratio of the frequency divider 68 is changed by the frequency-division control signal BD at certain time intervals continuously or discontinuously, the frequency of the oscillating signal Fs is also changed at predetermined time intervals. Because the frequency of the oscillating signal Fs as a switching signal is varied continuously or discontinuously, the fundamental frequency components and harmonic frequency components of the switching noise are dispersed. In this way, the amount of noise per unit time at a certain frequency can be reduced, whereby the influence of the generated noise can be reduced to practically acceptable levels.
However, in the examples shown in FIGS. 1 and 2, a special control circuit, such as a CPU, is required for setting the dividing ratio of the frequency divider 64. In the example of FIG. 3, the switching signal is provided by the oscillating signal Fs obtained by dividing the oscillating signal Fu in the frequency divider 68 whose dividing ratio is changed at predetermined time intervals. As a result, the oscillating signal Fs has discrete frequencies. Further, the frequency of the oscillating signal Fs stays the same for a predetermined time. Thus, the oscillating frequency is affected by noise for a predetermined time when the frequency of the tuner unit corresponds to the fundamental frequency of the oscillating signal Fs or its harmonic frequencies.